This invention relates generally to barrier coatings, and more particularly to barrier coatings for use in high temperature, aqueous environments.
Ceramic materials containing silicon and alloys containing silicon have been proposed for structures used in high temperature applications, such as gas turbine engines, heat exchangers, internal combustion engines, and the like. These materials are particularly useful in gas turbine engines which operate at high temperatures in aqueous environments.
The desired lifetime for components in such turbine applications may be tens of thousands of hours at temperatures above about 1000° C., for example. However, the components are known to experience significant surface recession under exposure to high-temperature, aqueous environments. Volatile silicon-based gaseous species typically form at temperatures above 1000° C., which causes the surface of the components to recede. The rate of recession may be 0.254 mm (0.010 in) or more per 1000 hours, for example, depending on the combustion conditions such as temperature and water vapor concentration in the combustion gas. This rate is unacceptably high for many component lifetime requirements.
An environmental barrier coating can protect the component from direct water-vapor exposure and inhibit volatilization. One proposed solution is a three layer environmental barrier coating as described in U.S. Pat. Nos. 6,387,456 and 6,410,148, which includes a silicon bond coat, a mullite and barium strontium aluminosilicate (BSAS) intermediate layer, and a pure BSAS top layer. However, after exposure to temperatures above about 1200° C. for long periods of time, the recession rate of BSAS may be unacceptably high for extended lifetime requirements. Furthermore, chemical reactions occur resulting in consumption of the bond coat and intermediate coat, which may still reduce the component lifetime below the desired level.
These and other drawbacks are present in known systems and techniques.